Example for situation when corresponding index does not exist

When a table does not have an index on a column that can
serve as the fragment of the resultant index, the database server
estimates the cost of building the index fragment for the column,
compares this cost to rebuilding the entire index for all fragments
on the resultant table, and chooses the index build with the least
cost.

Suppose you create a fragmented table and index with the following SQL statements:

The statement CREATE INDEX idx2 on tb1(col2) is
NOT completely equivalent to UPDATE STATISTICS LOW FOR TABLE
tb1, because the CREATE INDEX statement does not update index-
level statistics for the preexisting index called idx1.

In the preceding example, table tb3 does not have an index
on column a that can serve as the fragment of the resultant
index idx1. The database server estimates the cost of building
the index fragment for column a on the consumed table tb3 and
compares this cost to rebuilding the entire index for all fragments
on the resultant table. The database server chooses the index build
with the least cost.